This invention relates to an improved scanning optical system suitable for use with image recording apparatuses such as laser printers.
Scanning optical systems of the type contemplated by the present invention are highly sensitive to "tilting errors ", or errors due to the inclination of reflecting faces of the deflector such as a polygonal mirror. A method commonly employed to compensate for such tilting errors is to focus rays of light and form an image in a sub-scanning cross section at a point near the deflector. The term "sub-scanning cross section" as used herein means a cross section that includes the optical axis and which is perpendicular to a main scanning cross section where rays of light are scanned with the deflector.
FIG. 7 is a simplified cross-sectional view of a conventional scanning optical system as taken in a sub-scanning direction. Divergent rays of light issuing from a semiconductor laser 1 serving as a light source are collimated into a parallel beam by means of a collimator lens 2. The parallel beam emerging from the collimator lens 2 passes through a planoconvex cylindrical lens 3 having power only in a sub-scanning cross section and forms a line image at a point near a reflecting face 4 of a polygonal mirror. The reflected beam passes through an f.theta. lens 5 to form a spot that is scanned over an object of interest in a main scanning direction.
The cylindrical lens 3 used in this prior art system is shown more specifically in FIG. 8 and the numerical values that characterize this lens are listed in Table 1 below. According to the data shown in Table 1, the second principal point H is located within the lens. The symbols used in Table 1 have the following definitions: f, the focal length at the wavelength 780 nm; TL, the distance from the first surface of the lens to the focal point F; fB, the distance from the last surface of the lens to the focal point F; r, the radius of curvature of a surface in the sub-scanning cross section; d, the lens thickness; n.sub.d, the refractive index of the lens at the d-line (.lambda.=588 nm); v.sub.d, the Abbe number; and n.sub.780, the refractive index of the lens at the wavelength 780 nm.
TABLE 1 ______________________________________ f = 50.00 mm; TL = 51.35; fB = 47.35 Surface No. r d n.sub.d .nu..sub.d n.sub.780 ______________________________________ 1 25.536 4.00 1.51633 64.1 1.51072 2 .infin. ______________________________________
The scanning optical system of the type described above which is customarily used with laser printers is such that the principal point of the f.theta. lens is in the sub-scanning cross section is located fairly close to the polygonal mirror, with the imaging magnification set at a large value. This presents the problem that if the optical characteristics of the lens system vary on account of various factors including temperature changes, the position where a beam spot is to be formed is also likely to change. Since the change in the spot forming position in the main scanning cross section can be neglected for practical purposes, astigmatism will occur to produce a deformed spot that is elongated in the sub-scanning direction. As a consequence, the resolution and hence the printing performance deteriorates, which certainly is a serious problem in laser printers that require particularly high precision.
In order to reduce the variation in the spot forming position that can be caused by temperature changes or other phenomena, it may be proposed that the imaging magnification be lowered by locating the principal point of f.theta. lens 5 at a point closer to the object to be scanned. In this arrangement, the distance from the point of light convergence to the principal point of the f.theta. lens is increased, so if the angle of convergence of rays emerging from the f.theta. lens is the same as in the case shown in FIG. 7, the angle of spread of rays that can be admitted into the f.theta. lens will decrease. Assuming the arrangement shown in FIG. 8 which is identical to the case shown in FIG. 7 as far as the light source 1, collimator lens 2 and the cylindrical lens 3 are concerned, the angle of spread of rays that can be admitted into the f.theta. lens is so small that the effective diameter of flux issuing from the collimator lens will decrease to attenuate the energy that can be taken from the light source.
To increase the ratio of energy to flux diameter, the energy of the light source may be increased. However, semiconductor lasers which are commonly used today with image recording apparatuses such as laser printers are only capable of insuring stable outputs up to powers of approximately 3 mW and their operation will become unstable if the power output is further enhanced.
In order to keep the magnifying power of the f.theta. lens as low as possible and yet to insure that the effective diameter of flux issuing from the collimator lens is the same as in the case shown in FIG. 7, it is necessary to use a cylindrical lens having an increased focal length as shown in FIG. 10 but then the distance between the point of light convergence and the cylindrical lens will unduly increase to produce a bulky overall system.